Climate models predict warming in the Arctic due to greenhouse gases, and in the
absence of direct measurements, warming over the Arctic sea ice might be inferred from a
lengthening of the summer melt season. This analysis of satellite-based passive microwave
data from 1979 to 1996 reveals an 8% per decade increase in the number of melt days per
summer.

The authors, mostly earth or life scientists, review biological (plant and insect)
data, glacial findings, and temperature records taken from mountainous regions. At high
elevations, the overall trends regarding glaciers, plants, insect ranges, and shifting
isotherms show remarkable internal consistency, and there is consistency between model
projections and the observed changes. Chemical and physical changes in the
atmospherecompounded by large-scale land use and land-cover changeshave begun
to affect biological systems. Discusses implications for public health as well as for
developing an interdisciplinary approach to the detection of climate change.

Analyses of temperatures from boreholes in previously forested areas in western Canada
disclose sudden increases of one to two degrees in ground surface temperature at the times
of deforestation at each site. A warming of the ground surface over a large area of
Central Canada, synchronous with the deforestation of southern Ontario and neighboring
regions in the nineteenth century, may be an example of climate change linked to the
widespread creation of agricultural lands.

A 227-year record shows generally increased growth over the past century, coinciding
with warmer spring temperatures in south coastal Alaska. The recent warming exceeds that
of prior centuries, extending back to A.D. 1600.

Results illustrate two features of general interest. First, a city with constant
population still shows a temperature trend due to changes in urban morphology and energy
consumption. Second, the urban temperature trend effect varies in different parts of the
city, so a typical two-station approach (airport-urban) may be misleading.

For most U.S., Argentine and Australian cities, the yearly mean urban-to-rural
temperature difference is negatively correlated with rural temperature. This means that
regional data sets including urban records may have a bias associated with temperature
trends, as well as a bias due to urban growth. For the U.S. during 1901-1984, the two
types of bias could be of the same order but opposite sign.

"Effects of
Land Use on the Climate of the United States," G.B. Bonan (NCAR, POB 3000, Boulder CO
80307),Clim. Change, 37(3), 449-486, Nov. 1997.

Simulations with a land surface process model coupled to an atmosphere general
circulation model show that the climate of the U.S. with modern vegetation is
significantly different from that with natural vegetation. Important climate signals
include a 1° C cooling over the east and 1° C warming over the west in spring; summer
cooling of 2° C in the central region, and near-surface moistening in spring and summer.
The climate change caused by land use practices is comparable to other anthropogenic
climate forcings.

Evidence has appeared lately in the literature supporting both sides of the theory that
sulfate aerosols have suppressed greenhouse warming more in the Southern Hemisphere than
the Northern Hemisphere. This study, based on data sets of mean surface temperature over
land and sea for the past 140 years, shows no evidence of any such difference.

A compilation of paleoclimatic records from lake sediments, trees, glaciers, and marine
sediments shows that from 1840 to the mid-20th century, the Arctic warmed to the highest
temperatures in four centuries. This warming ended the Little Ice Age in the Arctic, and
has caused retreats of glaciers, melting of permafrost and sea ice, and alteration of
terrestrial and lake ecosystems. Although the warming, particularly after 1920, was likely
caused by increased atmospheric trace gases, the initiation of warming in the mid-19th
century suggests that increased solar radiance, decreased volcanic activity, and feedbacks
internal to the climate system also played roles.

Isolates two times over the 1979-1996 period when shifts occur in the compared data
sets: in 1981 and 1991. Both occur at times of satellite changes, but both could also be
entirely natural, being for instance related to volcanic eruptions. A problem with the
satellite data seems more likely, but if this can be ruled out, it implies a significant
change in lower atmospheric lapse rate on a global scale, particularly since 1991.

Observations support the view of Williams (1992) that monitoring of global lightning
activity could provide a thermometer-independent measure of temperature changes associated
with climate variability.

Investigates this approach and finds that thawing of Alpine lakes is determined to a
large extent by synoptic-scale meteorological processes. Modulation of incident radiation
by volcanic stratospheric aerosols may also strongly affect the timing of break-up.

In this correspondence, Christy et al. formally dispute claims by Hurrell and Trenberth
that the satellite record of temperature trend does not show any warming because of
problems calibrating instruments. (See Global Climate Change Digest, Apr. 1997, for
paper by Hurrell and Trenberth and related News item.)

A detailed analysis of trends in daily maximum and minimum air temperatures shows a
decrease in daily temperature range in most areas of the Arctic, but no evidence of any
greenhouse warming over the period.

Applies a statistical approach that does not require a hypothesis about the character
of the trend being sought, as is the case in previous analyses. Finds a trend of about
0.6° C per century since 1900 in both hemispheres. Extrapolation suggests that the
warming will be slowed or at least checked by the end of this century.

This detailed study for the period 1961-1993 shows generally mixed and weak regional
trends, although some patterns are identified. Results indicate that the direction of
change in regional temperature variability, unlike that of temperature itself, may be
difficult to predict even if changes in the broadscale atmospheric circulation are
evident.